The present invention is related to a blade for axial flow fans which contains innovations on its shape in such a way that it produces a lower noise level and a higher efficiency than the fan blades available in the prior art.
Axial flow fans have large application on many industry branches where it is necessary to move any gas, for example the air. Among the many components that constitute a fan, the blades are the elements that greatly influence its efficiency and noise level. The design of the other fan components that include, for example, the fixing structures and the fall body, demands a relatively small effort and it is well known in the prior art. Therefore, great attention should be given to the blade design in order to obtain a fan with the desired characteristics of noise level and efficiency.
There are many axial flow fan blades available in the prior art that contain some sort of improvement intended to reduce their noise level and to increase their efficiency. In U.S. Pat. No. 4,089,618 and No. 5,603,607, for example, fan blades with trailing edges containing notches or in a sawtooth shape are presented, and in U.S. Pat. No. 5,275,535 both the leading and the trailing edge are notched. Moreover, in U.S. Pat. No. 5,326,225 and No. 5,624,234 fan blades with planform shape curved forward and backward are presented. Furthermore, WO 95/13472 presents twisted fan blades with airfoil shaped sections.
Despite of the fact that these referred patents actually present a reduction on the noise level and an increase on the efficiency, the improvement obtained is quite modest. Furthermore, the inventive step present in these patents cannot be applied to all types of fan blade. Hence, the trailing edge with notches or in a sawtooth shape in U.S. Pat. No. 4,089,618 and No. 5,603,607, for example, only results in improvement in blades with very thin aerodynamic profile or in blades formed by a curved sheet. Consequently, the applicability of these patents is limited.
The noise produced by a fan blade comes from two main sources. The first source is the passage of the blade, during its rotational movement, over obstacles like the motor supports. Each time the blade passes over an obstacle it produces a pressure variation on the obstacle which results in noise emission, and the frequency of this noise is equal to the fan rotating frequency multiplied by the number of blades. This type of noise can be minimized by an adequate choice of the number of blades and by the design of the obstacles close to the blade rotation plane and, therefore, it will not be discussed in the present patent. The second noise source is the blade vortex emission. Vortexes are emitted at the blade trailing edge due to production of lift, as it is well known from the classical aerodynamics theory. Moreover, the vortex emission also occurs when there is flow separation over the blade. These emitted vortexes produce pressure variations which produce the noise. In opposition to the noise produced when the blades pass over obstacles, the noise produced by vortex emission does not present itself in only one frequency, but in a broadband related to the size of the emitted vortexes.
The present invention, then, presents technical innovation in the shape of a fan blade that results in a lower vortex emission and, therefore, in a reduction on the noise level produced by the fan. A lower vortex emission also implies in a lower amount of the energy provided to the fan being spent in the vortex production, such that a greater amount of energy can be used to produce work in the fluid. Hence, the reduction in the noise level comes with an increase in the fan efficiency.